Since the invention of the integrated circuit (IC), the semiconductor industry has experienced rapid growth due to continuous improvements in the integration density of various electronic components (e.g., transistors, diodes, resistors, capacitors, etc.). For the most part, this improvement in integration density has come from repeated reductions in minimum feature size, which allows more components to be integrated into a given area.
These integration improvements are essentially two-dimensional (2D) in nature, in that the volume occupied by the integrated components is essentially on the surface of the semiconductor wafer. Although dramatic improvements in lithography have resulted in considerable improvement in 2D IC formation, there are physical limits to the density that can be achieved in two dimensions. One of these limits is the minimum size needed to make these components. Also, when more devices are put into one chip, more complex designs are required.
In an attempt to further increase circuit density, three-dimensional (3D) ICs have been investigated. In a typical formation process of a 3D IC, two dies are bonded together and electrical connections are formed between each die and contact pads on a substrate. For example, one attempt involved bonding two dies on top of each other. The stacked dies were then bonded to a carrier substrate and wire bonds electrically coupled contact pads on each die to contact pads on the carrier substrate. This attempt, however, requires a carrier substrate larger than the dies for the wire bonding.
More recent attempts have focused on through vias (TVs), such as through silicon vias. Generally, a TV provides connectivity from one side of a substrate to another side of the substrate, thereby allowing semiconductor devices on a first side to electrically communicate with devices on a second side of the substrate. TVs, however, are difficult to test at both the die level and the wafer level.
Generally, chips (before and/or after dicing) are probed with test devices to determine the “known good dies.” Only the known good dies are used in subsequent processing, e.g., packaging. In the case of TVs, testing typically required probing both sides of the substrate in order to ensure that the electrical characteristics of the TVs are within acceptable limits. Probing both sides of the substrate, however, requires complicated equipment, time consuming, and expensive.